Steve, To me it looks like you have to break into the circuit to find out if the control loop from VRFDET is working.
A suggestion would be to lift one end of C121 to stop drive to Q6,Q7 and Q8 and prevent fuse popping. It should then be possible with a small DC voltage via a pot to simulate the DC generated on the VRFDET line. If you have a scope you can trace the RF back from Q5 and see why it does not vary when either the VRFDET voltage or the power control is varied. Hope this helps. Good luck. 73 Stewart G3RXQ On Thu, 25 May 2006 10:25:56 -0600, Steve Banks wrote: > Hi Everyone, > > The subject SN 4401 K2 belongs to a new ham, KC0TBO, who recently asked me > to complete and align it since he was moving out of state, and he needed a > hand with it. The rig was essentially all finished along with its > accessories (KSB2, KAT2, KNB2, KIO2). It appears to be very carefully > assembled and soldered. > > Since I had just completed the A-to-B mods on my own K2 (SN1599), and had > also completed a KX1 (SN267), I gladly agreed to lend him a hand finishing > up his K2. (My own K2's recent A-to-B upgrades posed a problem or two, as > did its original assembly, but they were minor compared to what I'm seeing > with SN 4401's TX problem.) > > THE PROBLEM: K2 4401's receiver and menu functions all seem to work FB. > CAL FIL, CAL PLL, CAL CUR, etc. all work exactly as expected. However, the > rig REFUSES to transmit at less than 5.1A of total current draw (and 19.1 > watts). Its 40m TX alignment procedure (p. 75 of the Rev. F manual) fails > totally, since putting the subject K2 in TUNE simply pops 3A and 5A fuses > from my Rigrunner's 13 VDC supply rail where both of my own Elecraft rigs > usually reside. That 5.1A TX current problem occurs irrespective of where > the K2 POWER control is set (full CCW at the moment). > > Scott King and Gary Surrency at Elecraft have offered numerous helpful > suggestions, and several mailings of parts, in an effort to remedy this K2's > TX current draw problem. What I've done is summarized below. I hope one of > you will help light my lights before I send the rig off to Elecraft for > repair. The past several weeks of trouble-shooting on my part have been > fruitless. > > THINGS TRIED SO FAR: > > 1. ON THE RF BOARD" As a first step, I carefully looked over and reflowed > the Low-Pass Filter toroids, assuming that an open- or short-circuit was > built in somewhere to the RF path to J4. I found no such problem. The > toroids all looked fine and reflowing their joints produced no difference in > the TX problem. I also removed, rewound and reinstalled T1, T2, T3 and T4 > at Gary's and Scott's suggestion, thinking that one or more of them had a > damaged winding. I replaced Q7, Q8, Q11, and Q12, since the incessant 5.1 A > load only showed up in the TUNE mode when the 12V line is applied to the > Power Amplifier. (I'm pretty sure it will also show up in any other > key-down situation.) > > While Q7 and Q8 were removed I also measured the base bias on both of the FA > transistors, substituting a 10-ohm base-pad resistor as a "dummy-base-load," > again at Gary's suggestion. Both bias voltages were spot-on measuring 0.63 > VDC each in TUNE.) I'm satisfied now that Q7 and Q8 are not damaged. I > also replaced C137 and C138 as possible gremlins just in case the TX base > bias had been way too high and was destroying Q7 and Q8. RFC8 and RFC9 were > also replaced. Again, no improvement was seen in the TX current problem. > > 2. ON THE DISPLAY BOARD: I replaced the POWER control pot, on a just-in-case > basis. I also found and reflowed one or two LCD pins to correct an > intermittent segment on the display mode indicator...no big deal, but it was > easily fixed. > > 3. On the Control Board I reflowed a number of joints, thinking that maybe > the U6 socket, the MCU, or both, were damaged or open. I replaced Q11, > thinking maybe it was damaged and failing to offer a proper ISENSE signal to > Pin 2 of the MCU. But still no difference: 5.1A in the TUNE mode. > > 4. Both the 8V and 5V regulator outputs were measured. Results were 8.01 > and 4.98 VDC respectively. The idling (DISPLAY) current in the RX mode > seems fine at 0.22ma with the S-meter in DOT and the LCD backlight turned > off. That current increases only a few ma with the BAR mode set and the LCD > turned ON. The BFO delivers a good range of bandwidth at 5.40 KHz. > > Sorry for the bandwidth, but if any of you have any suggestions for where > else to look for the 5.1A TX current problem, I'd sure like to hear about > them, either on or off the reflector. > > 73, > > Steve Banks > K0PQ > K2 S/N 1599 > KX1 S/N 00267 > (and temporary caretaker of K2 4401) > > > _______________________________________________ > Elecraft mailing list > Post to: Elecraft@mailman.qth.net > You must be a subscriber to post to the list. > Subscriber Info (Addr. Change, sub, unsub etc.): > http://mailman.qth.net/mailman/listinfo/elecraft > > Help: http://mailman.qth.net/subscribers.htm > Elecraft web page: http://www.elecraft.com _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
